1. Field of the Invention
The present invention relates in general to a method for joining two metal plates by using so-called spot welding, and more particularly to such a joining method as above, which also assures sealing between the joined two metal plates.
2. Description of the Prior Art
In order to clarify the task of the present invention, one conventional method for sealingly joining two metal plates by using spot-welding and sealing members will be outlined with reference to FIG. 2, which method is widely employed in automobile makers.
Referring to FIG. 2, there are shown two metal plates 10 and 12 which are sealingly spot-welded at their mutually overlapped end portions 10a and 12a. The joining is achieved by taking the following steps.
First and second metal plates 10 and 12 are prepared, the plate 10 having at its one end portion 10a a laterally extending grooved ridge 10b which is press-formed. Then, the end portions 10a and 12a of the metal plates 10 and 12 are properly overlapped one over the other with an elongate seal bar 20 received in the groove 10b, that is, in a space 22 defined between the groove 10b and the end portion 12a of the second metal plate 12. The seal bar 20 is constructed of a nitrile rubber of a type which expands somewhat when heated. Spot-welding is then applied to given portions 24 of the mutually overlapped end portions 10a and 12a to assure tight joining between the two metal plates 10 and 12. Then, a sealant 26 in viscous condition is applied to the edge 10c of the first metal plate 10 having a longitudinally extending part thereof in contact with the end portion 12a of the second metal plate 12. The sealant 26 is made of a thermoplastic material, such as a polymer containing mainly a polyvinyl chloride.
Then, the metal plates 10 and 12 thus joined are entirely coated with suitable paint and put in an oven for drying the paint. The drying process is carried out for about 30 minutes at a temperature between 160.degree. C. and 170.degree. C.
During this drying process, the seal bar 20 expands somewhat thereby filling the space 22 and thus assuring a sealing at the positions adjacent the seal bar 20. At the same time, the sealant 26 is cured.
When the drying process terminates, the joined two metal plates 10 and 12 are exposed to a room temperature atmosphere to cool. With this cooling process, the sealant 26 is solidified thereby achieving a sealing at the positions near the edge 10c of the first metal plate 10.
However, due to its inherency, the following drawbacks are encountered in the above-mentioned conventional joining method.
First, for the sealing, there are employed two sealing means, viz., the seal bar 20 and the sealant 26, and there is a need to provide the first metal plate 10 with the grooved ridge 10b. These induce a troublesome production process.
Second, when the joined two metal plates 10 and 12 are subjected to a considerable torsional stress, the solidified sealant 26 tends to peel off thereby deteriorating the sealing property thereof. This is because of the limited contact area of the sealant 26 relative to the first and second metal plates 10 and 12.